1. Field of the Invention
This invention relates generally to three dimensional (3D) sound. More particularly, it relates to a digital implementation of interaural time delays used in 3D digital sound applications.
2. Background of Related Art
Three-dimensional (3D) sound has become integral part of many personal computer (PC) and consumer electronics devices. It allows a user to experience realistic sound from any direction using only headphones or speakers.
The rendering of 3D sound involves simulation of a number of psychoacoustic phenomena occurring when sound is transmitted through air to each ear. Three of the most important phenomena are interaural time difference (ITD), interaural intensity difference (IID), and the head related transfer function (HRTF). The ITD is the difference in time that it takes for a sound wave to reach both ears. The IID is the sound level difference between each ear. The HRTF is the transfer function containing any filtering information about the transmission of sound to a particular ear. This impulse response contains information about the transmission of sound from a particular angular direction, including any reflections from the shoulder or head and any reflections occurring within the pinna of the ear.
ITD is an important and dominant parameter used in 3D sound rendering. The interaural time difference is responsible for introducing binaural disparities in 3D audio or acoustical displays. In particular, when a sound object moves in a horizontal plane, the interaural time delay is constantly changing depending on the relative location of the sound source and listener. Applying an accurate ITD to a sound can be used to create aural images of sound moving in any desired direction with respect to the listener.
Conventional 3D sound systems embed the interaural time difference in empirically determined HRTFs, typically determined with a mannequin head implanted with microphones in its ears. These delays typically have a relatively large resolution, e.g., 100 microseconds.
However, there are at least two basic problems with the implementation of the ITD in a digital environment. In a discrete time environment, time resolution is limited by sampling rate. The traditional use of integer sample delay has limitations. First, the ITD must be rounded to an integer delay, this gives less precision to the rendered ITD delay. Second, a 3D sound rendering which involves motion between multiple angles will incorporate different ITDs. In this situation there will be a discontinuity produced when the renderer switches between each ITD, thus, causing a ‘click’. There is thus a need for a method and apparatus for providing a smoothed perceptually ‘click-free’ 3D sound rendering of the ITD.